The present invention relates to sensing position and motion using sensor devices, and more specifically to sensing position and motion using optical encoder sensors.
Encoders are useful in many applications. In one application, computer interface devices such as joysticks, mice, track balls, steering wheels, etc. make use of encoders to determine the position of a user manipulatable object (manipulandum) in a workspace of the user object, and provide the position information to a host computer that is connected to the interface device. An encoder can be used to sense position of the manipulandum in one or more degrees of freedom. Haptic feedback interface devices are a form of interface device in which motion of the manipulandum is sensed and forces are output on the manipulandum or the device housing using actuators such as motors. Haptic feedback devices require accurate position sensing of a manipulandum to determine force output, especially force feedback devices which output forces on the manipulandum in its degrees of freedom.
Optical encoders use paired light sensors and light sources with mechanical interruptions to measure rotary or linear position. Rotary optical encoders are a widely-used form of digital sensor. Optical encoders used for sensing rotational motion typically include a spinning disk attached to a moving member such as a rotating motor shaft. For example, FIG. 1 is a diagram of an incremental rotary optical encoder 10 attached to a motor shaft 12 of a motor 14 to measure the angle of rotation of the shaft. A light source 16 emits a beam of electromagnetic energy toward a light sensor or detector 18 through an encoder disk 20 which is transparent or includes open slots. An encoder hub 22 couples the disk 20 to the shaft 12. Electronics 24 allows the necessary signals to pass to and from the source and detector. FIG. 2 shows the face of the optical encoder disk 20, with a striped pattern 26 of spokes or slits that provide periodic interruptions between the light source and light sensor as the motor shaft rotates. This creates a stream of pulses at the output of the detector.
The encoder 10 of FIG. 1 preferably uses two emitter-detector pairs (both emitters included in the light source 16 and both detectors included in the light sensor 18). If only one pair is used, the single output signal indicates motion, but cannot indicate which direction the encoder disk/shaft is turning. A practical encoder requires the addition of a second detector (and second emitter if appropriate) slightly offset from the first so that it produces a square wave pulse stream that is 90 (electrical) degrees out of phase with the first pulse stream, allowing the sensing of direction of motion, as is well known to those of skill in the art.
Optical encoders typically use LEDs to emit the light that passes through the encoder disk to the detector. While LEDs are an inexpensive, proven technology, they are far from ideal in many respects. The perfect optical encoder emitter would emit a bright, collimated light beam, and would require drive currents as low as practically possible. A bright emitter raises the signal-to-noise ratio of the sensor system. Collimated light projects a uniform pattern of shadows from the encoder disk that does not change size if the disk flutters closer to or farther from the emitter. Even distribution of light intensity simplifies the design of the detector. Low drive currents are especially important for battery-powered systems and devices running off of low-power buses such as those found in portable military and commercial systems.
Optical encoders use detectors to capture the light signals created by the emitter and rotating disk and convert them into electrical signals. After amplification and analog-to-digital conversion the TTL-compatible signal can be read by a microcontroller or data acquisition hardware. Two channel signals (one channel from each detector) are typical output of a ubiquitous quadrature optical encoder, using two photodetectors with channel signals offset by 90 degrees (xc2xc of the grid spacing interval on the encoder disk). High-resolution quadrature encoders, however, are typically large and expensive.
Optical encoder disks play a key role in determining the resolution and size of optical encoders. Higher resolution encoders require finer pitch grid patterns on the disks, or disks with larger circumferences which possess more grid lines per revolution. Manufacturers use various technologies to fabricate encoder disks. The cost and minimum grid pitch vary with the technologies. Most encoders use plastic film disks imprinted with standard lithographic methods. These disks can reliably accommodate a grid pitch as fine as 0.002xe2x80x3. The cheapest technology, molded plastic disks, can only achieve a grid pitch of 0.040xe2x80x3.
The performance of the optical encoder is determined by the characteristics of the emitter, the detectors, and the encoder disk. The required sensing resolution of the encoder largely determines the size of the encoder by influencing the size of the encoder disk. Making encoders smaller requires more precise fabrication of the encoder disk, better emitters, and more sophisticated detector arrays. However, any of these measures can become prohibitively expensive, especially for low-cost products in which the encoders are incorporated. For example, interface devices such as mice, joysticks, or the like could benefit greatly from smaller and more accurate optical encoders, but the cost of these devices must be kept low to remain competitive in the consumer market.
The present invention is directed to integrated optical encoder devices. The innovations herein can provide several components on an integrated circuit chip for enhanced performance and manufacturability.
More particularly, emitters, detectors, and encoders disks are described which can be easily produced with semiconductor processes. A detector array, for example, can be provided with lens to collimate light and enhance detection.
In another aspect of the present invention, an integrated sensor processing device detects motion of a moving member. The sensor device includes an array of a plurality of photodetectors that receive energy from a beam of electromagnetic energy emitted from an emitter, each photodetector outputting a signal indicating a detection of energy from the beam. Also included are at least one analog-to-digital converter, at least one state machine, a counter, and a communication module that provides a position to a controller. The integrated sensor processing device is provided on a single integrated circuit chip. Preferably, the communication module includes digital circuitry for providing serial data to the controller, which can be a microprocessor or other circuitry. The sensor processing device can be included in a haptic feedback device coupled to a host computer.
In other embodiments, the sensor integrated circuit chip also includes a sensor processing unit coupled to the counter and determining a velocity of the moving member, and a force computation unit coupled to the sensor processing unit and determining a force to be output by at least one actuator coupled to the force feedback computation unit. In yet other embodiments, digital logic circuitry on the chip is coupled to the analog-to-digital converters and selects one of the photodetector signals to provide a higher resolution than the other signals.
In another aspect of the present invention, a sensor device detects motion of a moving member and includes an emitter that emits a beam of electromagnetic energy, a moving member having an encoder pattern, an array of photodetectors that receive energy from the beam, and a mode selection circuit that can select a high resolution mode in which each detector provides the signal independently to permit a higher sensing resolution. The mode selection circuit can also select a low resolution mode in which at least two of the photodetectors are considered grouped and where the signals output by the grouped photodetectors are combined to achieve a lower sensing resolution than in the high resolution mode, where less power is consumed by the sensor device in the low resolution mode. For example, an emitter control circuit can control the emitter to output the beam for less power in low resolution mode, e.g. to pulse the beam with a greater off-time in low resolution mode than in high resolution mode, and/or to lower the intensity of the beam.
In another aspect of the present invention, a detector for an optical encoder includes a first array of photodetectors that receive energy from a beam emitted by an emitter of the optical encoder, where the first array is used for absolute sensing of the moving member, and a second array of photodetectors is used for incremental sensing of the moving member, and where the first array and second array of photodetectors are included on a single semiconductor integrated circuit chip. An encoder disk can modulate the energy from the emitted beam onto the first array and the second array, and can include a Gray code pattern for modulating light onto the first array and a slot pattern for modulating light onto the second array.
The present invention advantageously provides an optical encoder including many components that enhance the sensing performance of the encoder and which are integrated on a single integrated circuit chip for easier manufacturability. For example, semiconductor emitters, focussing lens, absolute and incremental detector arrays, sensor processing circuitry, and force determination circuitry can all be provided on a chip. In addition, the low resolution and high resolution modes can provide enhanced functionality for a sensor chip.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following specification of the invention and a study of the several figures of the drawing.